Conventional ultraviolet (UV) spectroscopy represents one of the least expensive methods for measuring component concentrations of a chemical process stream. However, conventional UV technology requires significant dilution of the process stream and many chemical components such as NaOH and Na2CO3 do not absorb ultraviolet radiation in the wavelength range currently measurable by conventional UV spectroscopy.
To overcome the dilution problems associated with utilizing conventional UV spectroscopy equipment, those skilled in the art have employed attenuated total reflectance (ATR) probes for analyzing process streams. Such methods are disclosed in Chai et. al., Spectrophotometric In-Line Monitoring of the Electrochemical Production of Polysulfides Using an ATR-Probe, Process Control and Quality, Vol. 11, No. 2, p. 153 (1998); Schlemmer et. al., ATR technique for UV/Vis Analytical Measurements, Fresenious Z Analytical Chemistry, Vol. 329, pp. 435-439 (1987); and Chai, Process Analytical Chemistry Applied to Liquors in the Pulping Industry, Swedish Centre for Process Analytical Chemistry, The Royal Institute of Technology, S-100 44, Stockholm, Sweden, ISBN 91-7179-653-4. However, ATR probes employed by the prior art are incapable of providing useful absorbency data below 210 nm which limits their application to process streams containing chemical components which absorb ultraviolet light above 210 nm.
In particular, this significant limitation prevents those skilled in the art from simultaneously determining the three major components found in paper mill kraft liquor streams. Two key components of kraft liquor streams, NaOH and Na2CO3, could not heretofore be individually detected because NaOH and Na2CO3 do not absorb ultraviolet light above 210 nm. Therefore, NaOH and Na2CO3 are commonly considered a single component referred to in the prior art as non-absorbing components or part of the total non-absorbing salt concentration. Because NaOH and Na2CO3 are non-absorbing above a wavelength of 210 nm the component concentrations of NaOH and Na2CO3 cannot be analyzed separately and determined individually.
Accurate determination of NaOH and Na2CO3 concentrations is critical to the operation of many kraft mill processes. In kraft pulping, lignocellulosic material, e.g., wood chips, is treated with an aqueous liquor containing active pulping chemicals, referred to as a white liquor. The white liquor typically contains sodium hydroxide and sodium sulfide, the two active cooking chemicals.
The fibers that make up the wood chips are separated from one another when the lignin that binds them together is dissolved in the white liquor at an elevated temperature. Once the fibers have been separated, the spent cooking liquor is collected. This spent pulping liquor is referred to as black liquor. The concentrated black liquor is burned in a chemical recovery furnace where the sulfur compounds are reduced to sodium sulfide. The inorganic ash recovered from this process is then dissolved in water producing what is referred to as green liquor. The green liquor contains primarily sodium sulfide and sodium carbonate. The green liquor can be converted into white liquor, by contacting the green liquor with calcium hydroxide in water. This process converts sodium carbonate (Na2CO3) into sodium hydroxide (NaOH) and is referred to as recausticizing.
The rate and selectivity of delignification in the pulping process is strongly affected by the quality of the white liquor. The quality of the white liquor is defined by the concentrations of the sodium hydroxide and the sodium sulfide (Na2S). High quality white liquor has high concentrations of NaOH and Na2S, and low concentrations of Na2CO3 and sodium sulfate (Na2SO4). To control the pulping process, it is necessary to monitor and adjust the relative concentration of the major components. This control can only be achieved if accurate measurements can be taken in-situ, thus allowing the process operator to make timely adjustments to the concentrations.
Known sensors for white liquor analysis are based upon conductivity, Fourier-Transform infrared FTIR, near infrared (NIR) or conventional UV spectroscopy. However, sensors based upon conductivity, FTIR and conventional UV spectroscopy can only provide a single component measurement. For example, FTIR and conductivity only detect the EA concentration. Likewise, conventional UV can only provide sulfide concentration.
Prior art methods based upon NIR like that disclosed in U.S. Pat. No. 5,616,214 may provide a means for simultaneously measuring the component concentrations of a kraft liquor stream. However, the NIR equipment necessary to conduct the analysis disclosed in the '214 patent is prohibitively expensive.
While conventional UV spectroscopy equipment is considerably cheaper than NIR, it too has certain disadvantages. The primary disadvantage of conventional UV spectroscopy is that it requires very high dilution rates, on the order of 10,000, before the process liquor can be analyzed. Also, since dissolved oxygen can react with sulfide and therefore greatly affect the accuracy of the measurements, it has heretofore been necessary to deoxygenate the samples prior to evaluation with conventional UV spectroscopy.
Accordingly, of the three primary components of a kraft liquor stream whose concentrations are critical to kraft operations (NaOH, Na2CO3 and Na2S), only the concentration of Na2S could be determined using prior art techniques. While the combined concentration of the non-absorbing salts, NaOH and Na2CO3, can individually be determined using the prior art methods, this total non-absorbing salt concentration cannot be used to effectively control kraft mill operations.
Thus, there exists a need for a rugged, inexpensive, simple analysis method and apparatus that can be used directly in the processing line, or in an associated flow cell that will provide the individual concentrations of all the major components within a kraft liquor stream without dilution of the liquor sample.
The inventive system provides individual concentration information for NaOH, NaCO3 and Na2S in real time, without dilution of the liquor sample. Thus, the present invention allows for immediate concentration adjustments resulting in improved kraft mill operation. The analyzer of the present invention can be installed directly in a pulping liquor stream as a probe or can be incorporated as part of a sample system as a flow cell. The analyzer of the present invention costs significantly less than competing infrared technologies and is simple to operate.
Further advantages of the invention will be set forth in part in the description which follows and in part will be apparent from the description or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.